The present invention relates to a method of operating a TDMA mobile station so as to transmit speech in a return path channel corresponding to the better of two forward path channels.
In communication systems, particularly those where one station is mobile, the quality of signals received from a channel can vary from time to time, sometimes dropping to the point where a signal becomes unusable due to noise or even disappears. A known solution to this problem is to transmit the same information over two or more channels. The likelihood of the signals in all of the channels being unusable at the same time is much lower than the likelihood of one channel being unusable. The likelihood is further reduced if the physical properties, e.g. carrier frequencies or paths, of the channels differ significantly.
In is known for a base station and a mobile system to transmit in two or more manners. Since substantially the same power must be radiated for each manner of transmission in such a system as would be radiated for the single manner used in a simple system, the power requirements of the stations are increased when diversity is employed. With modern low-power signal processing circuitry, the extra power required by the station""s rf power amplifier can almost double the power consumption of a station as a whole even when using only two-fold diversity. This problem is particularly acute in the case of battery powered equipment such as handheld mobile telephones.
According to the present invention, there is provided a method of operating a TDMA mobile station so as to transmit speech in a return path channel corresponding to the better of two forward path channels, the method including:
during one frame""s duration, digitizing user speech and storing the resultant digitized user speech data;
during one frame""s duration, coding digitized user speech data, produced in the preceding frame and storing the encoded digitized user speech data;
tuning to receive bursts from two paths and then demodulating the bursts;
decoding the demodulated bursts to produce time-compressed, error-corrected audio data and a value for the BER (bit error rate) of each said burst;
comparing the BER values to determine the lower at the end of a frame;
expanding the time-compressed audio data, received in the preceding frame to which the lower BER relates
converting the expanded audio data into an analogue signal and generating an acoustic signal in dependence thereon; and
transmitting the the encoded digitized user speech data via the return path corresponding to the forward path followed by the burst having the lower BER in the preceding frame.
Preferably, the user speech is digitized by a DSP module provided in the mobile station.
Preferably, the digitized user speech data is coded by a DSP module provided in the mobile station.
Preferably, said tuning is performed by outputting from a controller, provided in the mobile station, frequency control data to a frequency sythesizer, provided in the mobile station.
Preferably, the decoding is performed by a DSP module provided in the mobile station.
Preferably, said comparing of BER values is performed by a controller provided in the mobile station.